This invention relates to inspecting pipelines for cracks and, in particular, to inspecting pipelines using magnetic flux leakage inspection principles.
The increasing occurrence of stress corrosion cracking (SCC), especially in North America, and increased regulatory requirements creates a need for pipeline operators to identify pipelines with occurring SCC and identify the extent and severity of SCC. There is a justifiable reluctance to utilize sophisticated and costly inspection technologies on pipelines where little or no SCC has be identified thru normal maintenance operations (cut outs, corrosion investigations and others). On some pipeline sizes there is no available technology to survey the pipelines using the sophisticated technologies, due to size restrictions, tight bends, product type or speed, and other considerations.
SCC is typically found on pipelines, such as high pH (e.g., pH 9-13) SCC (also referred to as ‘classical SCC’) generally appears as intergranular cracking whereby a thick oxide layer forms on the pipe in a heavily concentrated carbonate-bicarbonate soil environment. Load variances on a pipe can cause crack tip strain that results in destruction of the oxide layer and further extension of the crack. A newer form of SCC, near-neutral pH (e.g., 5-7) SCC (also referred to as non-classical SCC), appears as transgranular cracking and is usually caused by groundwater containing CO2 which originates from sources such as disbonded pipeline coatings. Resulting cracks may be further aggravated by cyclical loads associated with the pipelines due to the stress ratios placed on these loads. Other elements present in the environment, such as hydrogen, have also been found to contribute to the susceptibility of SCC.
Pipeline operators and owners maintain integrity management plans (IMPs) for addressing procedures for maintaining pipelines. The procedures provide processes and recommended tools for performing routine maintenance, assessments, and corrective activities for ensuring the continued operation of the pipeline, as well as for ensuring environmental and public safety relating to these operations. Existing pipeline inspection procedures can be expensive, invasive, and laborious. For example, determining SCC by physical inspection often requires extensive excavation of a pipe and manual examination by the human eye. Further, many existing tools and processes for pipe inspection address or uncover one or more specific types of pipe defects or are geared toward a specific type of pipeline, and are not equipped to handle the variety of known issues, defects, and pipeline types that are in operation today. It is desirable to provide a way to manage pipeline conditions that covers a variety of operational conditions without incurring undue expense and labor.